The received signal energy in a wireless system varies widely depending on transmitted power, multipath effects, and losses that occur during transmission. At the same time, the signal processing blocks within a receiver require a fairly precise amount of energy in order to produce the best performance. Thus it is essential that a wireless receiver include an automatic gain control (AGC) mechanism.
Here we describe a novel AGC method for the multiband orthogonal frequency division multiplexed (OFDM) system described in one of the physical layer standards proposed for IEEE 802.15.3a Personal Area Networks. The proposed standard comes from the WiMedia Alliance and can be found on their website: www.wimedia.org. Some aspects of the WiMedia standard are described here to aid understanding of the present invention.
In the WiMedia standard, packets of data for radio transmission include a 30-symbol preamble, a 12-symbol header, and a payload of variable length containing user information. Each symbol within a packet has duration 312.5 ns. The standard allows for multi-user transmission by defining a number of frequency bands and frequency hopping patterns that use those bands. As an example, the mandatory portion of the standard specifies 3 frequency bands as follows:
TABLE IBand NumberFrequency Range (GHz)13.168-3.69623.696-4.22434.224-4.752
The standard allows for 7 hopping patterns so that multiple users can utilize these 3 bands simultaneously. The hopping patterns are as follows:
TABLE IITFCFrequency Hopping in Pattern (band numbers)1123123213213231122334113322511111162222227333333
For example, for time frequency code (TFC) equal to 1, the first symbol of a packet is sent in band 1, the second symbol in band 2, the third symbol in band 3, the forth symbol in band 1, and so on. The patterns shown repeat every 6 symbols.
There are several challenges associated with designing an AGC for the particular system in the WiMedia standard. First, the initial AGC adjustments need to be accurate. Second, they need to occur as quickly as possible and within the preamble of the packet so that all operations that depend upon gain—such as channel equalization—achieve optimum performance.
As such, a new automatic gain control is needed.